The present invention generally relates to image data reading apparatus, and more particularly to an image data reading apparatus which uses the peak level of an image signal as a reference voltage for a quantization part to convert the image signal into digital information.
Conventionally, an image data reading apparatus projects a light from a light source to a surface of an original sheet carrying image information for reading the image information, and a reflected light from the original sheet surface is photoelectrically converted into an analog signal indicative of the image information. And, undesired shading is eliminated from this analog signal by a correction circuit of the image data reading apparatus. This corrected analog signal from which the undesired shading is eliminated is quantized into digital information (usually binary code) by a quantization circuit of the image data reading apparatus, and this digital information is outputted from the image data reading apparatus to an image forming unit.
Such a conventional image data reading apparatus is disclosed, for example, in Japanese Laid-Open Patent Application No. 55-61170. In a case of this conventional image data reading apparatus, before a leading edge of an original sheet reaches a reading position within the image data reading apparatus, a white reference signal obtained from a light reflected back from a white reference plate is detected and stored within the image data reading apparatus. As the original sheet is transported to the reading position, such a white reference signal previously stored is used as a reference level for the correction circuit to eliminate undesired shading from an image signal obtained from a reflected light from the original sheet carrying image information. After the image signal is thus corrected, a peak level of the image signal is detected and stored by a peak level holding circuit of the image data reading apparatus, and this peak level stored is used as a reference level for a quantization circuit to convert the image signal into digital information. In addition, the peak level holding circuit always performs, even after the white reference signal is received by the light receiving part, a detection and holding of a peak level of the image signal until an image signal processing is completed with the reflected light from the original sheet.
However, due to the shape of a leading edge of the original sheet transported along the transport path, a light from the light source is sometimes regularly reflected back from the original sheet, and this regularly reflected light is received by the light receiving part. The peak level of output signal from the light receiving part in such a case is unusually high, and the peak level holding circuit detects and stores such a very high peak level of the output signal from light receiving part, because the peak level holding circuit is continuously in operation. The peak level of the output signal in this case is much higher than an ordinary peak level of the output signal which is obtained from a background region within the original sheet. Based on such a very high peak level of the output signal which is used as a reference level, the quantization circuit converts an image signal into digital information. If an image is reproduced from this digital information, the resulting image would be very dark as a whole, and therefore it is difficult to quantize accurately an image signal into digital information, the quantization being carried out based on the reference level which is unusually high due to regular reflection of light at a leading edge of an original sheet.